Voltage to current converter with negative feedback

ABSTRACT

A voltage/current converter includes a first differential pair including a first and second NPN transistor, a second differential pair including a third and fourth NPN transistor, a first resistor disposed between the emitters of the transistors of the first differential pair, a second resistor disposed between the emitters of the transistors of the second differential pair, third and fourth resistors disposed between the bases of the first and third transistors and of the second and fourth transistors, respectively, and a voltage negative feedback means disposed between the terminals of the second resistor and the bases of the transistors of the first differential pair in order to fix the voltage across the second resistor to a predetermined input voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to circuits providing a currentproportional to an input voltage, and more particularly to such acircuit that can be easily fabricated in the form of an integratedcircuit.

2. Discussion of the Related Art

FIG. 1 schematically shows a circuit conventionally used forvoltage/current conversion. Such circuit includes two NPN-type bipolartransistors, Q1 and Q2, having their collectors connected to a highvoltage VCC and their emitters connected to a low voltage VEE throughcurrent sources I. Emitters are interconnected through a resistor R1.The collector currents of these transistors are referenced IC1 and IC2.The voltage Ve to be converted into a current is applied between thebases B1 and B2 of transistors Q1 and Q2. Thus, the voltage V(R1) acrossR1 is:

    V(R1)=Ve (Vbe2-Vbe1).

The current ΔI in resistor R1 is ΔI=V(R1)/R1. The emitter current of Q1is I+ΔI, and the emitter current of Q2 is I-ΔI. Assuming that thebase-emitter voltage drops Vbe1 and Vbe2 of transistors Q1 and Q2 areequal, and that the difference of the emitter currents is equal to thedifference of the collector currents, then IC1-IC2=2ΔI=2Ve/RI.

A conventional circuit 10, for example of the current mirror type,provides the difference between currents IC1 and IC2 and provides at theoutput a current 2ΔI that is proportional to Ve.

In the above discussion, two simplified assumptions have beenconsidered. The first assumption is that Vbe1=Vbe2; the secondassumption is that the difference in the emitter currents is equal tothe difference of the collector currents. In fact, the collectorcurrents of transistors Q1 and Q2 are IC1=α(I+ΔI) and IC2=α(I-ΔI), whereα is the common-base current gain of transistors Q1 and Q2 (it isassumed that the two transistors have the same gain). α is expressed asa function of the common-emitter current gain, β, of transistors Q1 andQ2 by the relation α=1/(1+1/β). Thus, current ΔI, instead of beingsimply equal to Ve/R1 is in fact expressed by the relation:

    ΔI=α[Ve+(Vbe2-Vbe1)]/R1.

Thus, a first error is due to the value of Vbe2-Vbe1 that is non-zeroand varies with the current and temperature. The second error is due tothe fact that α is not equal to 1, and the variation of α can benon-negligible when β is relatively low. For example, if β=35, α isequal to 0.97; so, the proportion error is approximately 3%.

A conventional manner to correct the first error is illustrated in FIG.2. A negative feedback is imposed between the emitter and the base ofeach transistor Q1 and Q2 through differential amplifiers A1 and A2whose outputs are applied to the bases of transistors Q1 and Q2,respectively, and whose inverting inputs are connected to the emittersof transistors Q1 and Q2, respectively. The input signal is appliedbetween the non-inverting inputs E1 and E2 of amplifiers A1 and A2.Thus, the error associated with the non-zero value of Vbe2-Vbe1 isattenuated by a factor AV, where AV is the voltage gain of eachamplifier A. This gain can be very high, for example higher than 100,and the slight error in the difference of the base-emitter voltages thusbecomes an error of the second order, substantially lower than 1:1000.

A conventional method to correct the second error is to substitute aDarlington-type circuit for each transistor Q1 and Q2. As is shown inFIG. 3, each transistor Q1 and Q2 is associated with a respectiveDarlington-connected transistor Q'1 and Q'2. Considering thattransistors Q1, Q2, Q'1 and Q'2 have the same common-emitter currentgain, β, the common-base current gains α become 1[1+1/β(β+1)]. Again, anerror of the first order is changed into an error of the second order,that becomes negligible.

Thus, the circuit of FIG. 3 that combines the correction means of thefirst and second type of error is satisfactory and provides currents IC1and IC2 whose difference is proportional (the proportion ratio beingassociated with the value of resistor R1) to the difference in voltagebetween terminals E1 and E2.

However, this circuit has a drawback inherent in the presence of aDarlington-type circuit. Indeed, when considering a single transistorsuch as transistor Q1 or Q2, its collector/emitter voltage drop at theconducting state, Vsat, is approximately 0.2 volt. Whereas, in the caseof a Darlington circuit, this voltage drop is increased by the meanvalue of a base-emitter voltage drop, that is, the total voltage dropapproaches a value of approximately 1 volt. This phenomenon isaggravated at low temperatures where Vbe increases. This relatively highvoltage drop across the transistors is a drawback especially due to thefact that, as illustrated in FIG. 1, there are additional transistorstages between the high supply voltage and the low supply voltage(between VCC and VEE). This requires that the difference VCC-VEE must behigher in the case of the circuit of FIG. 3 than in the case of thecircuit of FIG. 1 and does not allow the use of the circuit of FIG. 3when this voltage difference (supply voltage) is low.

SUMMARY OF THE INVENTION

An object of the invention is to provide a voltage/current converter inwhich the voltage drop across the elements providing the current is aslow as possible.

To achieve this object, the invention provides a circuit for convertinga voltage into a current difference, including a first differential pairincluding a first and second NPN transistor whose emitters are connectedto a low supply terminal through a current source and whose collectorsare connected to a high supply terminal, a second differential pairincluding a third and fourth NPN transistor whose emitters are connectedto a low supply terminal through a current source and whose collectorsare connected to a high supply terminal, a first resistor disposedbetween the emitters of the transistors of the first pair, a secondresistor disposed between the emitters of the transistors of the secondpair, third and fourth resistors disposed between the bases of the firstand third transistors and of the second and fourth transistors,respectively, and a voltage negative feedback means disposed across thesecond resistor and the bases of the transistors of the first pair inorder to fix the voltage across the second resistor to a predeterminedinput voltage.

According to an embodiment of the invention, the negative feedback meansincludes operational amplifiers, each having a first input connected toa terminal of the second resistor, a second input forming an inputterminal, and an output applied to the base of a transistor of the firstpair.

According to an embodiment of the invention, each operational amplifieris formed by a fifth NPN transistor and a sixth PNP transistor, the baseof the fifth transistor being connected to a terminal of the secondresistor, the emitter of the fifth transistor being connected to theinput terminal, the collector of the fifth transistor being connected tothe high supply terminal through a current source, the base of the sixthtransistor being connected to the collector of the fifth transistor, theemitter of the sixth transistor being connected to the high supplyterminal through a current source and to the base of a transistor of thefirst differential pair, and the collector of the sixth transistor beingconnected to the low supply terminal.

According to an embodiment of the invention, an operational amplifier isformed by an NPN transistor whose collector is connected to the highsupply terminal through a current source and to the base of a transistorof the first pair, having its emitter connected to a second inputterminal and its base connected both to a first input terminal and aterminal of the second resistor.

According to an embodiment of the invention, the third and fourthresistors have a same value, equal to half the value of the secondresistor.

The foregoing and other objects, features, aspects and advantages of theinvention will become apparent from the following detailed descriptionof the present invention when taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1-3, above described, are useful for illustrating the state of theart;

FIG. 4 schematically shows a first embodiment of the circuit accordingto the invention operable for converting a voltage into a current;

FIG. 5 represents a variant of the circuit of FIG. 4; and

FIGS. 6 and 7 represent in more detail implementations of negativefeedback amplifiers used in FIGS. 4 and 5, respectively.

DETAILED DESCRIPTION

FIG. 4 schematically shows an embodiment of a circuit according to theinvention. The circuit includes a first differential pair includingtransistors Q1 and Q2 and a second differential pair includingtransistors Q3 and Q4. In the represented embodiment, all thetransistors are of the NPN-type. The emitters of transistors Q1-Q4 areconnected to the low supply terminal VEE through current sources I. Thecollectors of transistors Q1-Q4 are connected to a high supply terminalVCC, directly or indirectly but in both cases through paths having thesame impedance for each transistor of a differential pair. The emittersof transistors Q1 and Q2 are connected through a resistor R1 and theemitters of transistors Q3 and Q4 are connected through a resistor R2.The outputs of the operational amplifiers A1 and A2 are connected to thebases of transistors Q1 and Q2, respectively. The inverting inputs ofamplifiers A1 and A2 are connected to the emitters of transistors Q3 andQ4, respectively, and the non-inverting inputs E1 and E2 of amplifiersA1 and A2 correspond to input terminals between which is applied thevoltage that is to be converted into a current, the converted currentbeing proportional to the difference between the collector currents IC1and IC2 of transistors Q1 and Q2. The bases of transistors Q1 and Q3 andthe bases of transistors Q2 and Q4 are respectively connected throughconnecting resistors Rb1 and Rb2 having an equal value, Rb.

For the sake of simplification of the description, references 11 and 12designate the base terminals of transistors Q1 and Q2, 13 and 14designate the emitter terminals of transistors Q3 and Q4, and 15 and 16designate the collector terminals of transistors Q1 and Q2. Terminals11-16 are the input/output terminals of a block surrounded by dashedlines and referenced 20 in FIG. 4.

In the case of FIGS. 2 and 3, because of the presence of the high gainoperational amplifiers A1 and A2, voltage Ve appears across resistor R1between inputs E1 and E2. Similarly, in the example of FIG. 4, voltageVe appears across resistor R2. Current IR2 in resistor R2 is equal toVe/R2 virtually without error or with a negligible error of the secondorder. Currents I+IR2 and I-IR2 appear again in the emitters oftransistors Q3 and Q4. The differential voltage between the bases oftransistors Q1 and Q2 is also differentially applied to the bases oftransistors Q3 and Q4, with a resulting voltage difference acrossresistor R1 (neglecting the errors in differences of Vbe that are, inthat case, of the second order):

    V(R1)=Ve+2Rb.IR2/β

or

    V(R1)=Ve(1+2Rb/βR2).

The difference in the collector current of transistors Q1 and Q2 isequal to twice the current value in resistor R1 multiplied by the gainratio α (≈1-1/β). Thus, by selecting Rb=R2/2, one obtains:

    IC1-IC2=(2Ve/R1) (1+1/β) (1-1/β)

that is,

    IC1-IC2=(2Ve/R1) (1-1/β.sup.2)

that is, the 1/β² term being negligible, this current difference becomessubstantially equal to 2Ve/R1 and, as desired, is proportional tovoltage Ve. This result is obtained whereas the voltage acrosstransistors Q1 and Q2 remains the voltage drop at the conduction stateof a bipolar transistor that is, as indicated above, approximately 0.2volt.

Like in conventional circuits, the proportion ratio between the voltageand the current depends only on the value of resistor R1. The values ofresistors R2, Rb1 and Rb2 intervene only by their ratio that can beextremely accurate in an integrated circuit device.

Of course, those skilled in the art can devise various modifications tothe circuit according to the invention.

FIG. 5 shows a variant in which the connection mode of the operationalamplifier A1 and A2 is modified. FIG. 5 shows again the block 20 of FIG.4 with its terminals 11-16. In FIG. 5, the inverting inputs of theoperational amplifiers A1 and A2 are connected to terminals 13 and 14through respective resistors R3 and R4, and the non-inverting inputs areinterconnected. The input terminals E1 and E2 then correspond to theinverting input of amplifier A1 and to the non-inverting inputs ofamplifiers A1 and A2. In this case, a voltage can be applied betweeninputs E1 and E2 with interposition of a series resistor on terminal E1in a manner known by those skilled in the art.

FIG. 6 shows an exemplary embodiment of operational amplifiers used forthe circuit of FIG. 4. Amplifier A1 only will be described in detail,since amplifier A2 is identical. Amplifier A1 is formed by an NPN-typetransistor Q5 and a PNP-type transistor Q6. The collector of transistorQ5 is connected to the high supply terminal through a current source 15.The emitter of transistor Q6 is connected to terminal 11 and to the highsupply terminal through a current source 16. The base of transistor Q6is connected to the collector of transistor Q5. The collector oftransistor Q6 is connected to the low supply terminal VEE. The base oftransistor Q5 is connected to terminal 13. The emitter of transistor Q5corresponds to the input terminal E1.

FIG. 7 shows an exemplary embodiment of the operational amplifiers A1and A2 used in a circuit as the one shown in FIG. 5. The operationalamplifiers A1 and A2 are formed by NPN-type transistors Q7 and Q8,respectively. The collectors are respectively connected to terminals 11and 12 and to the high supply terminal through respective currentsources 17 and 18. The emitters are interconnected and form the inputterminal E2. The bases of transistors Q7 and Q8 are respectivelyconnected to terminals 13 and 14 through resistors R7 and R8. Asindicated above, the input terminal E1 is preferably connected to thebase of transistor Q7 through a resistor R9.

Having thus described one particular embodiment of the invention,various alterations, modifications, and improvements will readily occurto those skilled in the art. Such alterations, modifications, andimprovements are intended to be part of this disclosure, and areintended to be within the spirit and scope of the invention.Accordingly, the foregoing description is by way of example only and isnot intended as limiting. The invention is limited only as defined inthe following claims and the equivalents thereto.

What is claimed is:
 1. A circuit for converting a voltage into a currentdifference, including:a first differential pair including first andsecond NPN transistors each having an emitter coupled to a low supplyterminal through first and second current sources, respectively, andeach transistor having a collector coupled to a high supply terminal; asecond differential pair including third and fourth NPN transistors eachhaving an emitter coupled to the low supply terminal through third andfourth current sources, respectively, and each having a collectorcoupled to the high supply terminal; a first resistor disposed betweenthe emitters of the transistors of the first differential pair; a secondresistor disposed between the emitters of the transistors of the seconddifferential pair; third and fourth resistors disposed between bases ofthe first and third transistors and of the second and fourthtransistors, respectively; and voltage negative feedback means disposedbetween the terminals of the second resistor and the bases of thetransistors of the first differential pair for fixing the voltage acrossthe second resistor to a predetermined input voltage.
 2. The circuit ofclaim 1, wherein the negative feedback means includes operationalamplifiers, each having a first input connected to a terminal of thesecond resistor, a second input forming an input terminal, and an outputconnected to the base of one of the transistors of the firstdifferential pair.
 3. A circuit for converting a voltage into a currentdifference, including:a first differential pair including first andsecond NPN transistors each having an emitter coupled to a low supplyterminal through first and second current sources, respectively, andeach transistor having a collector coupled to a high supply terminal; asecond differential pair including third and fourth NPN transistors eachhaving an emitter coupled to the low supply terminal through third andfourth current sources, respectively, and each having a collectorcoupled to the high supply terminal; a first resistor disposed betweenthe emitters of the transistors of the first differential pair; a secondresistor disposed between the emitters of the transistors of the seconddifferential pair; third and fourth resistors disposed between bases ofthe first and third transistors and of the second and fourthtransistors, respectively; and a voltage negative feedback meansdisposed between the terminals of the second resistor and the bases ofthe transistors of the first differential pair for fixing the voltageacross the second resistor to a predetermined input voltage; wherein thenegative feedback means includes first and second operational amplifiersand fifth and sixth resistors, the first operational amplifier having afirst input connected to a first terminal of the second resistor via thefifth resistor, the second operational amplifier having a first inputconnected to a second terminal of the second resistor via the sixthresistor, the first and second operational amplifiers each having asecond input connected to an input terminal and an output connected tothe base of one of the transistors of the first differential pair. 4.The circuit of claim 3, wherein the negative feedback means furtherincludes a seventh resistor, and wherein the second input of the firstoperational amplifier is connected to one of the input terminals via theseventh resistor.
 5. A circuit for converting a voltage into a currentdifference, including:a first differential pair including first andsecond NPN transistors each having an emitter coupled to a low supplyterminal through first and second current sources, respectively, andeach transistor having a collector coupled to a high supply terminal; asecond differential pair including third and fourth NPN transistors eachhaving an emitter coupled to the low supply terminal through third andfourth current sources, respectively, and each having a collectorcoupled to the high supply terminal; a first resistor disposed betweenthe emitters of the transistors of the first differential pair; a secondresistor disposed between the emitters of the transistors of the seconddifferential pair; third and fourth resistors disposed between bases ofthe first and third transistors and of the second and fourthtransistors, respectively; and a voltage negative feedback meansdisposed between the terminals of the second resistor and the bases ofthe transistors of the first differential pair for fixing the voltageacross the second resistor to a predetermined input voltage; wherein thenegative feedback means includes operational amplifiers, each having afirst input connected to a terminal of the second resistor, a secondinput forming an input terminal, and an output connected to the base ofa transistor of the first differential pair; and wherein eachoperational amplifier is formed by a fifth NPN transistor and a sixthPNP transistor, the base of the fifth transistor being connected to aterminal of the second resistor, the emitter of the fifth transistorbeing connected to the input terminal, the collector of the fifthtransistor being connected to the high supply terminal through a currentsource, the base of the sixth transistor being connected to thecollector of the fifth transistor, the emitter of the sixth transistorbeing connected to the high supply terminal through a current source andto the base of the transistor of the first differential pair, and thecollector of the sixth transistor being connected to the low supplyterminal.
 6. A circuit for converting a voltage into a currentdifference, including:a first differential pair including first andsecond NPN transistors each having an emitter coupled to a low supplyterminal through first and second current sources, respectively, andeach transistor having a collector coupled to a high supply terminal; asecond differential pair including third and fourth NPN transistors eachhaving an emitter coupled to the low supply terminal through third andfourth current sources, respectively, and each having a collectorcoupled to the high supply terminal; a first resistor disposed betweenthe emitters of the transistors of the first differential pair; a secondresistor disposed between the emitters of the transistors of the seconddifferential pair; third and fourth resistors disposed between bases ofthe first and third transistors and of the second and fourthtransistors, respectively; and a voltage negative feedback meansdisposed between the terminals of the second resistor and the bases ofthe transistors of the first differential pair for fixing the voltageacross the second resistor to a predetermined input voltage; wherein thenegative feedback means includes operational amplifiers, each having afirst input connected to a terminal of the second resistor, a secondinput forming an input terminal, and an output connected to the base ofa transistor of the first differential pair; and wherein one of theoperational amplifiers is formed by a further NPN transistor having acollector connected to the high supply terminal through a current sourceand to the base of one of the transistors of the first differentialpair, having an emitter connected to a second input terminal and havinga base connected both to one of the first input terminals and to one ofthe terminals of the second resistor.
 7. The circuit of claim 6, whereinthe negative feedback means further includes a fifth resistor, andwherein the base of the further transistor is connected to the one ofthe first input terminals via the fifth resistor.
 8. The circuit ofclaim 1, wherein the third and fourth resistors have a same value thatis equal to half the value of the second resistor.
 9. A circuit forconverting a voltage into a current difference, including:a firstdifferential pair including first and second NPN transistors each havingan emitter coupled to a low supply terminal through first and secondcurrent sources, respectively, and each transistor having a collectorcoupled to a high supply terminal; a second differential pair includingthird and fourth NPN transistors each having an emitter coupled to thelow supply terminal through third and fourth current sources,respectively, and each having a collector coupled to the high supplyterminal; a first resistor disposed between the emitters of thetransistors of the first differential pair; a second resistor disposedbetween the emitters of the transistors of the second differential pair;third and fourth resistors disposed between bases of the first and thirdtransistors and of the second and fourth transistors, respectively; anda voltage negative feedback circuit disposed between the terminals ofthe second resistor and the bases of the transistors of the firstdifferential pair.
 10. The circuit of claim 9, wherein the negativefeedback circuit includes operational amplifiers, each having a firstinput coupled to a terminal of the second resistor, a second inputforming an input terminal, and an output coupled to the base of one ofthe transistors of the first differential pair.
 11. A circuit forconverting a voltage into a current difference, including:a firstdifferential pair including first and second NPN transistors each havingan emitter coupled to a low supply terminal through first and secondcurrent sources, respectively, and each transistor having a collectorcoupled to a high supply terminal; a second differential pair includingthird and fourth NPN transistors each having an emitter coupled to thelow supply terminal through third and fourth current sources,respectively, and each having a collector coupled to the high supplyterminal; a first resistor disposed between the emitters of thetransistors of the first differential pair; a second resistor disposedbetween the emitters of the transistors of the second differential pair;third and fourth resistors disposed between bases of the first and thirdtransistors and of the second and fourth transistors, respectively; anda voltage negative feedback circuit disposed between the terminals ofthe second resistor and the bases of the transistors of the firstdifferential pair; wherein the negative feedback circuit includes firstand second operational amplifiers and fifth and sixth resistors, thefirst operational amplifier having a first input coupled to a firstterminal of the second resistor via the fifth resistor, the secondoperational amplifier having a first input coupled to a second terminalof the second resistor via the sixth resistor, the first and secondoperational amplifiers each having a second input coupled to an inputterminal and an output coupled to the base of one of the transistors ofthe first differential pair.
 12. The circuit of claim 11, wherein thenegative feedback circuit further includes a seventh resistor, andwherein the second input of the first operational amplifier is coupledto one of the input terminals via the seventh resistor.
 13. A circuitfor converting a voltage into a current difference, including:a firstdifferential pair including first and second NPN transistors each havingan emitter coupled to a low supply terminal through first and secondcurrent sources, respectively, and each transistor having a collectorcoupled to a high supply terminal; a second differential pair includingthird and fourth NPN transistors each having an emitter coupled to thelow supply terminal through third and fourth current sources,respectively, and each having a collector coupled to the high supplyterminal; a first resistor disposed between the emitters of thetransistors of the first differential pair; a second resistor disposedbetween the emitters of the transistors of the second differential pair;third and fourth resistors disposed between bases of the first and thirdtransistors and of the second and fourth transistors, respectively; anda voltage negative feedback circuit disposed between the terminals ofthe second resistor and the bases of the transistors of the firstdifferential pair; wherein the negative feedback circuit includesoperational amplifiers, each having a first input coupled to a terminalof the second resistor, a second input forming an input terminal, and anoutput coupled to the base of a transistor of the first differentialpair; and wherein each operational amplifier is formed by a fifth NPNtransistor and a sixth PNP transistor, the base of the fifth transistorbeing coupled to a terminal of the second resistor, the emitter of thefifth transistor being coupled to the input terminal, the collector ofthe fifth transistor being coupled to the high supply terminal through acurrent source, the base of the sixth transistor being coupled to thecollector of the fifth transistor, the emitter of the sixth transistorbeing coupled to the high supply terminal through a current source andto the base of a transistor of the first differential pair, and thecollector of the sixth transistor being coupled to the low supplyterminal.
 14. A circuit for converting a voltage into a currentdifference, including:a first differential pair including first andsecond NPN transistors each having an emitter coupled to a low supplyterminal through first and second current sources, respectively, andeach transistor having a collector coupled to a high supply terminal; asecond differential pair including third and fourth NPN transistors eachhaving an emitter coupled to the low supply terminal through third andfourth current sources, respectively, and each having a collectorcoupled to the high supply terminal; a first resistor disposed betweenthe emitters of the transistors of the first differential pair; a secondresistor disposed between the emitters of the transistors of the seconddifferential pair; third and fourth resistors disposed between bases ofthe first and third transistors and of the second and fourthtransistors, respectively; and a voltage negative feedback circuitdisposed between the terminals of the second resistor and the bases ofthe transistors of the first differential pair; wherein the negativefeedback circuit includes operational amplifiers, each having a firstinput coupled to a terminal of the second resistor, a second inputforming an input terminal, and an output coupled to the base of atransistor of the first differential pair; and wherein one of theoperational amplifiers is formed by a further NPN transistor having acollector coupled to the high supply terminal through a current sourceand to the base of one of the transistors of the first differentialpair, having an emitter coupled to a second input terminal and having abase coupled both to one of the first input terminals and to one of theterminals of the second resistor.
 15. The circuit of claim 14, whereinthe negative feedback circuit further includes a fifth resistor, andwherein the base of the further transistor is coupled to the one of thefirst input terminals via the fifth resistor.
 16. The circuit of claim9, wherein the third and fourth resistors have a same value that isequal to half the value of the second resistor.
 17. A circuit forconverting a voltage into a current difference, including:firstdifferential pair means having input terminals and output terminals, forproviding a current difference on the output terminals that isproportional to a voltage supplied on its input terminals, the firstdifferential pair means having a pair of first current gains; seconddifferential pair means coupled to the input terminals of the firstdifferential pair means, the second differential pair means having apair of second current gains; and voltage negative feedback meanscoupled to the input of the first differential pair means and to thesecond differential pair means, for applying the pair of second currentgains to reduce an error caused by the first pair of current gains beingof an insufficiently high value.
 18. The circuit of claim 17 wherein thefirst differential pair means includes:first and second NPN transistorshaving emitters coupled to a low supply terminal through a currentsource, having collectors coupled to a high supply terminal, and havingbases form the input terminals to the differential pair means, and afirst resistor disposed between the emitters of the transistors of thefirst differential pair.
 19. The circuit of claim 17 wherein the seconddifferential pair means includes:first and second NPN transistors havingemitters coupled to a low supply terminal through a current source andhaving collectors coupled to a high supply terminal, a first resistordisposed between the emitters of the transistors of the seconddifferential pair, and second and third resistors disposed between thebases of the first and second transistors and the input terminals of thefirst differential pair.
 20. The circuit of claim 19 wherein the voltagenegative feedback means are coupled to the terminals of the firstresistor in order to fix the voltage across the first resistor to apredetermined input voltage.
 21. The circuit of claim 19, wherein thesecond and third resistors have a same value that is equal to half thevalue of the first resistor.
 22. The circuit of claim 19, wherein thenegative feedback means includes operational amplifiers, each having afirst input coupled to a terminal of the first resistor, a second inputforming a circuit input terminal, and an output coupled to one of theinputs of the first differential pair means.
 23. The circuit of claim19, wherein the first NPN transistor provides a first current gain,wherein the second NPN transistor further provides a second currentgain, and wherein the voltage negative feedback means is also forapplying the third current gain to reduce the impact of differencesbetween the first pair of current gains on the relationship between thevoltage at the input terminals of the first differential pair means andthe current difference at the output terminals of the first differentialpair means.
 24. A circuit for converting an input voltage to adifferential output current, comprising:first through fourthtransistors, all of the same bipolar type, each having a base, anemitter, and a collector, the collectors of the first and fourthtransistors supplying the differential output current; first throughfourth current sources, one for each of the first through fourthtransistors, respectively, each coupled between the emitter of one ofthe first through fourth transistors and a supply voltage; a firstimpedance coupled between the emitters of the first and fourthtransistors; a second impedance coupled between the emitters of thesecond and third transistors; a third impedance coupled between the baseof the first transistor and the base of the second transistor; a fourthimpedance coupled between the base of the third transistor and the baseof the fourth transistor; and a control circuit, coupled to the base ofat least one of the first and fourth transistors, for establishing avoltage across the second impedance responsive to the input voltage. 25.The circuit of claim 24, wherein the control circuit is a feedbackcircuit that controls the voltage across the second impedance.
 26. Thecircuit of claim 25, wherein the input voltage is differential andincludes first and second input signals, and the feedback circuitcomprises two independent feedback loops:a first feedback loop includinga first control circuit receives the first input signal and drives thebase of the first transistor to force a voltage at the emitter of thesecond transistor to follow the first input signal; and a secondfeedback loop including a second control circuit that receives thesecond input signal and drives the base of the fourth transistor toforce a voltage at the emitter of the third transistor to follow thesecond input signal.
 27. The circuit of claim 26, wherein the firstcontrol circuit is an operational amplifier.
 28. The circuit of claim26, wherein the second control circuit is an operational amplifier. 29.The circuit of claim 26, wherein the first control circuit comprises afifth transistor having the same bipolar type of the first throughfourth transistors, whose base is coupled to the emitter of the secondtransistor and whose collector is coupled to the base of the firsttransistor.
 30. The circuit of claim 26, wherein the second controlcircuit comprises a sixth transistor having the same bipolar type of thefirst through fourth transistors, whose base is coupled to the emitterof the third transistor and whose collector is coupled to the base ofthe fourth transistor.
 31. The circuit of claim 24, wherein third andfourth impedances each have a value that is substantially half the valueof the second impedance.
 32. The circuit of claim 24, fabricated as anintegrated circuit.
 33. The circuit of claim 24, wherein each of thefirst through fourth impedances is a resistor.
 34. The circuit of claim24, wherein each of the first through fourth transistors is of the NPNtype.
 35. A bipolar transistor circuit for generating an output currentin response to an input voltage, including compensation forsubstantially eliminating a dependence of the output current on atransistor current gain alpha of the bipolar transistors making up thecircuit, comprising:a first differential pair of bipolar transistors,the emitters of the transistors each being coupled to a supply voltagethrough a separate current source; first resistive means, bridging theemitters of the transistors of the first differential pair, forconducting a difference current responsive to a difference between theemitter currents of the transistors of the first differential pair;compensating means, coupled to the bases of the transistors of the firstdifferential pair, for generating a compensation signal that variesinversely with alpha; a second differential pair of bipolar transistors,the bases of which being coupled to the compensating means, the emittersof which each being coupled to the supply voltage through a separatecurrent source, and the collectors of which each providing a collectorcurrent, wherein a ratio of collector current to emitter current of thetransistors of the second differential pair varies directly with alpha;second resistive means, bridging the emitters of the transistors of thesecond differential pair, for conducting a difference current responsiveto a difference between the emitter currents of the transistors of thesecond differential pair; and means, coupled to the bases of the seconddifferential pair, for establishing a predetermined voltage across thesecond resistive means, so that the inverse proportionality with alphaof the compensation signal substantially cancels out the directproportionality with alpha of the ratio of collector current to emittercurrent of the transistors of the second differential pair, therebysubstantially eliminating a dependence of the collector currents of thetransistors of the second differential pair upon alpha.
 36. The circuitof claim 35, wherein the means for establishing includes means,responsive to the input voltage, for controlling the voltage across thesecond resistive means using feedback.
 37. The circuit of claim 36,wherein said means for controlling further comprises:means for sensing avoltage across the second resistive means; means for comparing thevoltage across the second resistive means with the input voltage togenerate an error signal; and means for driving the bases of the seconddifferential pair in a manner that tends to reduce the error signal. 38.A method for reducing a dependence of an output current of a bipolarvoltage-to-current converter upon a transistor current gain alpha, thebipolar voltage-to-current converter having a first differential pair ofbipolar transistors, the emitters of which being coupled togetherthrough a first resistor, and having a second differential pair ofbipolar transistors, the emitters of which being coupled togetherthrough a second resistor, the transistors of the first and seconddifferential pair having the current gain alpha, the method comprisingthe steps of:A) driving the bases of the transistors of the firstdifferential pair to generate a voltage across the first resistorresponsive to a difference between the emitter currents of thetransistors of the first differential pair; B) generating a compensatingsignal substantially equal to the voltage across the first resistordivided by alpha; C) applying the compensating signal to an input of thesecond differential pair to generate a voltage across the secondresistor responsive to a difference between the emitter currents of thetransistors of the second differential pair, the collectors currents ofthe transistors of the second differential pair being directlyproportional to the compensating signal times alpha, a ratio of thecollector currents to the voltage across the first resistor therebybeing substantially independent of alpha.
 39. The method of claim 38,further comprising the step of:D) controlling the voltage across thefirst resistor by applying feedback.
 40. The method of claim 38, whereinthe step B further includes generating a voltage that exceeds a voltageacross the first resistor by an amount proportional to a base current ofthe transistors of the first differential pair.